Inkjet printing systems are generally of two types: drop on demand (DOD) printing systems and continuous inkjet (CIJ) printing systems. Inkjet printing is a standard method for printing a colored or pigmented image onto a substrate wherein a stream of ink droplets containing pigment particles are directed from a printing device to a surface of a suitable receiver element or substrate. The direction of the stream of droplets is controlled electronically so as to cause printing the drops in a desired image or information on the substrate surface without requiring contact between the printing device and the surface to which the ink is applied. Objects comprising substrates to which inkjet printing is well suited include but are not limited to, containers for consumer products, currency, draft checks, envelopes, letterhead, documents of various types, identification cards, lottery tickets, bank cards, identification strips, labels, brochures, signage, and other well-known materials.
Drop-on-demand printing systems are widely used in home or consumer inkjet printers and slower consumer printers, both of which have been available for several decades. As the name implies, this type of inkjet printing uses a print head that ejects drops of ink only when signaled to do so by a digital controller.
CIJ printing systems generally comprise two main components, a fluid system (including an ink reservoir) and one or more print heads. Ink can be pumped through a supply line from the ink reservoir to a manifold that distributes the ink to a plurality of orifices, typically arranged in linear array(s), under sufficient pressure to cause ink streams to issue from the orifices of the print head(s). Stimulations can be applied to the print head(s) to cause those ink streams to form streams of uniformly sized and spaced drops that are deflected in a suitable manner into printing or non-printing paths. Non-printing drops can be returned to the ink reservoir using a drop catcher and a return line. Thus, in contrast to DOD printing systems, CIJ printing systems involve the use of a continuous stream of ink drops that are separated to discriminate between spaced printing drops and non-printing drops. For example, this discrimination can be accomplished by air deflection or by electrostatically charging the drops and passing the charged drops through an electric field. Charged drops are deflected by a charge field and can be caught and returned to the reservoir of ink. Uncharged drops are printed onto a substrate or receiver material.
In general, pigment-based inks can comprise a wide variety of organic or inorganic pigments that can be chosen depending upon the specific application and performance requirements for the printing system and desired printing results (for example, desired hue). For example, such organic or inorganic pigments can include but are not limited to, titanium dioxide or other white pigments, carbon black or other black pigments, red pigments, green pigments, blue pigments, orange pigments, violet pigments, magenta pigments, yellow pigments, and cyan pigments. Iridescent and metallic pigments can also be used for special optical effects.
White inks can be prepared using high refractive index particles of inorganic materials such as metal oxides including titanium dioxide. Such particles are most efficient at scattering actinic light with particle sizes of from 200 nm to 1000 nm. However, because of their density, such particles in this size range or larger do not form stable aqueous dispersions and will readily settle out even when a typical dispersant is included in the aqueous dispersions or formulations.
Aqueous white inks are described in U.S. Patent Application Publication 2014/0292902 (Kagata et al.) in which metal oxides are mixed with binder resins such as polyester resins, fluorene resins, or styrene-acrylic resins, organic solvents (polyols or pyrrolidones), and surfactants. The metal oxide particles are defined by using a specified relationship of a structural factor, porosity, particle diameter, and specific gravity with the average particle size being greater than 150 nm and less than 10,000 nm, and more preferably in the range of from 300 nm to 600 nm.
Other white inkjet ink compositions are described in U.S. Pat. No. 7,850,774 (Oriakhi) in which the white pigment dispersions display a bimodal distribution wherein at least 5 weight % of the particles have an average size equal to or greater than 100 nm and at least 25 weight % of the particles have an average size of equal to or less than 50 nm.
Still another aqueous inkjet ink composition is described in U.S. Patent Application Publication 2014/0288208 (Sasada et al.), which composition comprises particles of titanium dioxide having an average primary particle diameter of 200 nm or more, a water-soluble resin, and self-dispersing resin particles having an average particle diameter of 40 nm or less.
U.S. Patent Application Publication 2014/0123874 (Kabalnov et al.) describes a white inkjet ink having high index refraction index particles having a diameter of less than 100 nm and low refraction index particles such as emulsion particles having a diameter of greater than 100 nm.
U.S. Pat. No. 7,592,378 (Lin et al.) describes aqueous titanium dioxide slurries for inkjet processes in which the titanium dioxide pigment is dispersed using a combination of a graft copolymer and a block copolymer. The titanium dioxide particles are considered to have a more preferred particle size of from 100 nm to 500 nm.
A mixture of a styrene acrylic resin and a urethane resin is used as a fixing resin in white compositions according to U.S. Pat. No. 8,697,773 (Okuda et al.). In these compositions, the more preferred particle size is 200 nm to 400 nm.
To obtain stable dispersions of inorganic pigments such as titanium dioxide in the presence of dispersants, the pigment particle size should be below 200 nm, and even less than 100 nm. However, at a particle size that is less than the wavelength of actinic light, such inorganic pigment particles do not effectively scatter actinic light to create a dry “white” coating of desired opacity.
Thus, there is a need to create stable (well dispersed) aqueous dispersions and aqueous inkjet ink compositions that comprise titanium dioxide or other white pigments or even other colorants wherein the particle size is generally below 200 nm for effective stability in the aqueous dispersions and inks and for effective actinic light scattering when the particles aggregate into larger light scattering centers upon drying on a surface, thereby providing desired opaque images.